1 files changed, 29 insertions, 2 deletions

diff --git a/src/backend/storage/ipc/dsm.c b/src/backend/storage/ipc/dsm.c
index 983d0fa744f..d2b231f7895 100644
--- a/src/backend/storage/ipc/dsm.c
+++ b/src/backend/storage/ipc/dsm.c
@@ -454,9 +454,9 @@ dsm_set_control_handle(dsm_handle h)
  * Create a new dynamic shared memory segment.
  */
 dsm_segment *
-dsm_create(Size size)
+dsm_create(Size size, int flags)
 {
-	dsm_segment *seg = dsm_create_descriptor();
+	dsm_segment *seg;
 	uint32		i;
 	uint32		nitems;
 
@@ -466,6 +466,21 @@ dsm_create(Size size)
 	if (!dsm_init_done)
 		dsm_backend_startup();
 
+	/* 
+	 * If we've been instructed to return NULL when it's not possible to
+	 * register another segment, check whether we seem to be at the limit.
+	 * This allows us to avoid the overhead of creating a new segment only to
+	 * immediately destroy it again.  Since we don't take the lock here, the
+	 * value we read might be slightly stale, but the remote possibility of
+	 * an unnecessary failure here shouldn't trouble anyone too much.
+	 */
+	if ((flags & DSM_CREATE_NULL_IF_MAXSEGMENTS) != 0
+		&& dsm_control->nitems >= dsm_control->maxitems)
+		return NULL;
+
+	/* Create a new segment descriptor. */
+	seg = dsm_create_descriptor();
+
 	/* Loop until we find an unused segment identifier. */
 	for (;;)
 	{
@@ -496,9 +511,21 @@ dsm_create(Size size)
 
 	/* Verify that we can support an additional mapping. */
 	if (nitems >= dsm_control->maxitems)
+	{
+		if ((flags & DSM_CREATE_NULL_IF_MAXSEGMENTS) != 0)
+		{
+			dsm_impl_op(DSM_OP_DESTROY, seg->handle, 0, &seg->impl_private,
+						&seg->mapped_address, &seg->mapped_size, WARNING);
+			if (seg->resowner != NULL)
+				ResourceOwnerForgetDSM(seg->resowner, seg);
+			dlist_delete(&seg->node);
+			pfree(seg);
+			return NULL;
+		}
 		ereport(ERROR,
 				(errcode(ERRCODE_INSUFFICIENT_RESOURCES),
 				 errmsg("too many dynamic shared memory segments")));
+	}
 
 	/* Enter the handle into a new array slot. */
 	dsm_control->item[nitems].handle = seg->handle;